i9o6.] SEE— THE CAUSE OF EARTHQUAKES. 367 



time, say 30 minutes, and then follows the sea wave which has taken 

 form owing to the forcing up of the water over the volcano, thus 

 forming the crest, and its withdrawal from the shore, forming the 

 trough. 



The water heaved up gradually settles and the wave approaches 

 the shore often with a velocity of something like eight kms. per 

 minute and thus sweeps everything before it. It then oscillates 

 back and forth with period of some 60 minutes, and for a day or two 

 the sea may continue to be agitated with appalling violence, and the 

 wave propagated to the remotest parts of the earth. 



If an observer were to witness such an earthquake in a region 

 where the shore was steep and the sea of uniform depth, and should 

 note the time of the sea wave and the direction of the normal to 

 the wave front as it first returns, he would have a very approximate 

 means of locating the situation of the new submarine volcano. It 

 would lie on the normal to the circular wave front, and at a distance 

 corresponding to the time of arrival in a sea of the given depth. 



The interval r required for the oscillation of the wave being 

 known, the theory of the wave motion could be worked out by the 

 general formula^ for a wave of any length A and any depth of the 

 water k, 



2 ittXs ^ + I 

 ^ =-^^k : (l) 



£ ^ — I 



Perhaps it could be found with sufficient approximation by the more 

 special forms, in which the velocity becomes, when the wave is long 

 compared to depth of the water: 



V= Vgk, or V= ^^{k + E) (2k + E) ; (2)^ 



E being the height of the crest of the wave above the normal level 

 of the water. 



In this way we could find not only the distance of the eruption 

 from the observer, but the direction, so as to fix its place with con- 



^ Airy, " Tides and Waves," Art. 169. 



'Report of Committee of the Royal Society on the Krakatoa Eruption, 

 p. 94. 



